In electrophotographic processes, a latent electrostatic image formed on a photoreceptor is usually developed with a pigment-containing resin powder. The resulting visible image is transferred onto a receiving sheet and subsequently fixed on the receiving sheet by application of either heat or pressure, or both. Recently, there has been a growing need for more rapid and efficient image fixing. To meet this need, the use of heated rolls as a fixing means has become popular in many modern electrophotographic systems.
In the method of fixing images with heated rolls, it is effective to lower either the softening point or melt viscosity of the binder material used for the toner in order to realize rapid fixing with lower power consumption. However, with the binder polymers (binder resins) which are commonly used as principal binding materials for toners, a lowering of the softening point often results in an increase in the change of "caking" (or "blocking") during toner storage, which will lead to reduced flowability of the toner. If the melt viscosity of the binder polymer is reduced, offsetting phenomena (such as where the unfixed toner image adheres to the surface of a fixing roller, e.g., a heated roller) will occur and the change of image fouling is increased.
In order to avoid these problems, various ideas have been proposed: one is to prevent offsetting phenomena by supplying a mold releasing agent such as silicone oil to heating rollers; another is to increase the glass transition temperature of the principal binder polymer of the toner; still another approach is to prevent caking during storage by applying fine particles of an inorganic material such as silica, titania, alumina, or of an organic material such as PMMA (Polymethylmethacrylate), as an external additive (e.g., prevent caking by adding fine particles having a particle size smaller than the toner particles on the outer surface of the toner particles).
High-molecular weight resins have conventionally been used as binding materials in toners. To make toners, the resins are mixed with pigments (e.g., carbon black), charge control agents and any other necessary additives by a suitable method such as kneading, and the resulting mixture is ground into fine particles. In this case, the binder resin retains the pigments and other constituent materials, and the toner softens upon application of either heat or pressure, or both, during fixing, whereupon the toner particles are deposited on and affixed to the microscopically undulating surface (i.e., surface having roughness) of the receiving sheet.
The prior art methods, however, have various problems. In order to supply silicone oil to heated rolls, a feed mechanism is necessary, and this adds to the cost of the apparatus. Furthermore, maintenance work becomes necessary in association with oil replenishment. If the glass transition temperature of the binder resin is adjusted to too a high level, its softening point will be increased so as to make it difficult to achieve successful fixing at low temperatures. If the glass transition temperature of the binder resin is lowered too much, the toner becomes more prone to caking.